High brightness light emitting diode (LED) light sources are in high demand for challenging applications in machine vision. Prior art in the machine vision lighting field typically utilize tungsten or tungsten halogen, metal halide, and xenon arc lamps or more recently, systems incorporating pre-packaged high brightness LEDs. High intensity linear lighting is used to illuminate the field of view of line scan cameras to visualize objects including printed materials on high speed printers and a variety of manufactured products that travel by on a moving conveyor belt or platform for what is typically referred to as web inspection. Objects that have high specular mirror-like reflection, however, require high uniformity of the far field in addition to high near field uniformity. Applications of interest include inspection of liquid crystal displays (LCDs), semiconductor wafers, glass panels, and solar cells. To properly inspect these objects for defects, a beamsplitter is typically used to reflectively couple the light onto the object normal to its surface. The camera typically views the object normal to its surface in transmission through the beamsplitter. Thus, the camera is effectively viewing directly into the line source, which is known as Bright Field (BF) Imaging. Non-uniformity of the far field radiance, from the perspective of the illumination plane, is observed as non-uniformity of the image on the camera. Such non-uniformities reduce signal to noise ratio, that is, camera sensitivity and reduce camera dynamic range. High performance, low cost, compact, and reliable linear lighting with high uniformity both in the near and far field is required for these Bright Field illumination sources.
Until recently, the industry standard was tungsten halogen lamps coupled into typically glass fiber optic bundles arranged in a line and imaged to a line by use of a spherical cylindrical lens. Companies such as Schott Fiber Optics, Dolan Jenner, Volpi, Illumination Technology, and Fiberoptic Technology manufacture such products with a range of available intensities and line lengths. For example, a single twenty four inch long linear lighting system from Schott Fiber Optics is comprised of two separate tungsten halogen light boxes coupled to two one half inch glass fiber optic cables which in turn terminate into a single twenty four inch long length of fiber which images to the illumination plane by use of a cylindrical spherical rod lens. One of the most serious limitations of tungsten halogen technology is that the intensity of the lamps degrades at a fast rate, and for viewing of specular objects, they also are characterized by poor far field uniformity. At full power, tungsten halogen lamps only last between about 50 hours and 500 hours before their initial intensity has degraded by the order of 50% or the lamps fail by filament burn out. The cost of the lamps is not the primary concern, however. It is the cost of shutting down a line to replace the lamp that is primarily driving the need for LED based systems.
Prepackaged LEDs are defined as devices comprising an LED or LED array disposed on top of one or more thermally and electrically conductive materials each with an associated thermal impedance, electrical leads and thermal backplane that are then intended to be attached to yet another board with additional thermal impedance. Examples of prepackaged devices include the Luxeon™ and Rebel™ product lines sold by Philips, the Osram Dragon™ and Ostar™ product lines, and the CREE X-Lamp™ product line.